Jimmy Ortega, age 11, of Tucson, airs. for his question:

What exactly are sedimentary rocks?

Sediments begin as solid fragments suspended in water or some other liquid. They are made of heavier materials than the surrounding liquid and they tend to sink to the bottom. However, sediments tend to settle more slowly when the liquid is turbulent.

Tea leaves sink to the bottom of the pot, and so do gritty grains of come. These dregs are sediments that have settled down because they weigh more than liquid tea or coffee. If you shake the pot, they tend to float around. But as the liquid becomes calm, they settle down again. This sedimentary process goes on in nature on a grand scale. The process is slow but when aided by certain changes in the landscape, it adds massive layers of rock to the crust of the earth.

Sedimentary rocks begin as fragmentary solids suspended in lakes and rivers, swamps and seas. Dusty fragments build up silty deposits, especially where rivers meet the sea. All kinds of suspended fragments abound in the ocean and form sedimentary layers as they sift down to settle on the bottom. The ocean floor is carpeted with oozy layers of silty mud mixed with shells and decaying marine life.

Through the ages, the world climate changes and the earth's crust tends to shift. Shallow waters may dry up and coastal seas may recede. These events leave stretches of new land, carpeted with layers of sediments that settled under the e water. As the moisture evaporates from these sedimentary layers, they become solid, sedimentary rocks.

The nature of the sediments depends upon the suspended particles. Waters abounding in tiny protozoan shellfish gather limy deposits that become layers of sedimentary chalk. Limestone sedimentary rocks are formed from deposits of corals and mollusks, protozoa and various seaweeds. Gritty deposits of sand become cemented into sandstone. Deposits of silty clay become hard sedimentary rocks called shale.

Sediments . .    Shift down slowly and settle on the bottom in flat, even layers.

For this reason, sedimentary rocks tend to be flat and often arranged in layers like

Thick pages.    Their colors range from drab, gray brown shale to rainbow tinted sandstone.

Many sedimentary layers contain fossils of sizeable marine creatures, and chalk, made

Mostly of microscopic shells, is rated as a fossilized rock.

Suspended sediments are shifted by moving water and after millions of years form new rocks far from their origins. The delta at the mouth of the Mississippi Missouri becomes choked with silty deposits. Someday they may form shales containing dusty fragments which were carried by the great river from the prairies. Rivers gather debris from thousands of miles and dump their sediments into the seas, often to form new layers of sedimentary shale and sandstone.